Project Summary Multiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS) which is mediated by myelin reactive effector T cells. Experimental autoimmune encephalomyelitis (EAE) serves as an animal model of MS. Foxp3+ regulatory T cells (Treg) are T cells which, in healthy individuals, keep CD4+ T cells in check. In MS patients, regulatory T cells are not as effective as in healthy individuals, and CD4+ T cells have an effector/memory phenotype and produce IFN-g, IL-17 and GM-CSF. Although the mechanisms by which regulatory T cells control nave CD4+ T cells have been well described, it is not known how regulatory T cells control effector /memory Th1, Th17 cells in vivo during the course of CNS autoimmunity. In addition, whether Th1, Th17 and ThGM cells differentially affect the activity of Treg during EAE has not been determined. Using an adoptive transfer model of effector myelin specific Th1 and Th17 cells in mice lacking cytokine receptors (IL-6 receptor (IL-6R) or the glycoprotein 130 (gp130) in regulatory T cells, we have established that deletion of IL-6R expression in Treg enhances their capacity to control Th17 but not Th1 cells. In contrast, the lack of gp130 in Treg compromises their capacity to suppress Th1 but not Th17 mediated EAE. In this proposal, we will address whether the modulation of IL-6R and gp130 signaling in Treg affects their capacity to control ThGM cells. In addition, using a novel mouse line that allows for the tracking and the conditional deletion of GM-CSF+ T cells, we will address whether the deletion of GM-CSF+ T cells during the progression of EAE can affect Treg functions and disease course. Together, our experiments will further establish how Th1, Th17 and ThGM cells differentially affect the activity of Treg during EAE. The completion of this proposal might also bring new insight on the basis for MS and EAE heterogeneity and on the foundations for the diminished suppressive activity of Treg in EAE and MS.